3 ms·

This is a great "theorem", thank you! It's easy to determine divisibility by 2, 3, 5, and 11. 7² is also easy because it's a square. 7×13=91 is the only compos

This is a great "theorem", thank you! It's easy to determine divisibility by 2, 3, 5, and 11. 7² is also easy because it's a square. 7×13=91 is the only composite number under 100 that isn't caught by these rules.

_kst_ 5y ago
A trick for remembering that 91 is composite is that it's of the form x² - y² (specifically 10² - 3²), so it's (x+y)(x-y) or 13 * 7.
- canjobear 5y ago
  seems easier to remember "91 is composite"
- deleted 5y ago
  [deleted]